is that Lady Cora's father is Jewish. Her mother, Mrs. Levinson, never converted, and the Levinson children appear to have been raised in some Protestant church. I suspect that the rules were different for the rich, as ever.

would have to convert to become a Jew. Papa doesn't count.

would also apply to Rose and Atticus, though Lord Sinderby seems to want to stop them so he can have a nice Jewish daughter-in-law.

There is a Catholic parish in our area that is a national Italian parish. These were common throughout the U.S. as various nationalities immigrated from Catholic nations such as Poland and Italy. The early generations at this parish wanted their children to marry another Italian, later ones wanted them to marry a Catholic and now they just pray that their children marry before having children.

society as the US.
Although the ideal might have been to marry within one's faith, timing and locale had a lot to do with whom one married.
If in a community of many or mostly same faith, it would have been easier to find a mate of the same religion.
Often times this was not so.

Of course, marrying outside of one's faith may have been frowned on, both by family and by clergy, but that didn't always prevent nature from taking its course. Marriage was more a necessity in earlier times, when gender roles were more defined re working, housekeeping, cooking, etc.

Within my family, my Methodist grandfather married my Catholic grandmother, albeit in the priest's parlor, rather than in the sanctuary. I presume he also had to promise to raise his children Catholic, as was the rule in that time.
On the other hand, an earlier ancestor left the Catholic church and raised her children as Methodists, though her spouse remained Catholic until death. There are several other examples of interfaith marriages just within my relationship.

Bottom line, people are human with human wants and needs regardless of the rules and the era.